MLA
Curtis, William H. Envelope Signed by William H. Curtis. William H. Curtis Was a Quartermaster's Clerk in the 58th Regiment Illinois Volunteer Infantry. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 18. https://digital.library.emory.edu/purl/0752fqz734-cor.
APA, 6th edition
Curtis, W. H. Envelope signed by William H. Curtis. William H. Curtis was a quartermaster's clerk in the 58th Regiment Illinois Volunteer Infantry (Box 19, Folder 18) [Envelopes]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/0752fqz734-cor.
Chicago
Curtis, William H. “Envelope Signed by William H. Curtis. William H. Curtis Was a Quartermaster's Clerk in the 58th Regiment Illinois Volunteer Infantry.” Envelopes. Civil War Collection, n.d. Box 19, Folder 18. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/0752fqz734-cor.